YesodAuthEmail 无法推断出 m ~ HandlerFor site0 [重复]答案

【问题标题】：YesodAuthEmail could not deduce m ~ HandlerFor site0 [duplicate]YesodAuthEmail 无法推断出 m ~ HandlerFor site0 [重复]
【发布时间】：2018-11-21 08:07:21
【问题描述】：

我正在尝试添加

instance YesodAuthEmail App

到Yesod-Postgres 脚手架（yesod 1.6 版）并陷入编译错误。

相关代码为：

instance YesodAuth App where
     type AuthId App = UserId
     ....
     authPlugins :: App -> [AuthPlugin App]
     authPlugins app = [authOpenId Claimed []] ++ extraAuthPlugins
         where extraAuthPlugins = [ authEmail ]

instance YesodAuthEmail App where
    type AuthEmailId App = UserId

    afterPasswordRoute _ = HomeR

    addUnverified email verkey =
        runDB $ insert $ User email Nothing

我收到的错误是：

/home/justin/code/yesodemail/src/Foundation.hs:273:11: error: • Could not deduce: m ~ HandlerFor site0 from the context: MonadAuthHandler App m bound by the type signature for: addUnverified :: Yesod.Auth.Email.Email -> VerKey -> AuthHandler App (AuthEmailId App) .... Expected type: m (AuthEmailId App) Actual type: HandlerFor site0 (Key User)

根据类型，

getEmail :: AuthEmailId site -> AuthHandler site (Maybe Email) 
type MonadAuthHandler master m = (MonadHandler m, YesodAuth master, master ~ HandlerSite m, Auth ~ SubHandlerSite m, MonadUnliftIO m)
type AuthHandler master a = forall m. MonadAuthHandler master m => m a

我原以为这会编译。我误会了什么？

附：我已尝试包含所有相关内容，但完整的 Foundation.hs 位于 https://gist.github.com/hyperpape/39d4d2baf67d3bdbdba45a943e7e0425

【问题讨论】：

标签： haskell authorization yesod type-families

【解决方案1】：

runDB的类型是：

runDB :: YesodDB site a -> HandlerFor site a

要在AuthHandler 中调用它，您需要将其提升到HandlerFor。

如果我没记错的话，这就是来自MonadHandler 的liftHandler 方法的用途。

如果你用它编写runDB 电话，它应该可以工作：

addUnverified email verkey =
    liftHandler . runDB $ insert $ User email Nothing

我找到了您问题here的详细答案。

【讨论】：